An example of the inspection of a semiconductor wafer will be described. A semiconductor device is manufactured by repeating a step of transferring a pattern formed on a photomask onto a semiconductor wafer through lithography process and etching process. In a manufacturing process of a semiconductor device, the yield of the semiconductor device is largely influenced by quality of the lithography process, the etching process and others, generation of a foreign material, and the like. Therefore, in order to detect the generation of failures and defects in an early stage or in advance, a method for inspecting a pattern on a semiconductor wafer during the manufacturing process has been conventionally implemented.
As a method of inspecting a defect present in a pattern on a semiconductor wafer, a defect inspection apparatus that irradiates the semiconductor wafer with white light to compare equal circuit patterns of a plurality of LSIs by using an optical image has been put into practical use. Brief summary of the inspection system is described in “Monthly Semiconductor World” August, 1995, pp. 96-99 (Non-Patent Literature 1). Also, as inspection methods using an optical image, Japanese Patent Application Laid-Open Publication No. 3-167456 (Patent Literature 1) discloses a, system in which an image of an optically illuminated region on a substrate is formed by using a time-delay integrating sensor and the image and a design characteristic inputted in advance are compared to detect a defect, and Japanese Examined Patent Application Publication No. 6-58220 (Patent Literature 2) discloses a method in which image degradation at the time of image acquisition is monitored and the image degradation is corrected at the time of image detection, thereby performing a comparison inspection using a more stable optical image. When a semiconductor wafer is inspected in a manufacturing process by using such an optical inspection system, a residue and a defect of a pattern having a silicon oxide film or photosensitive photoresist material which is transmissive on its surface cannot be detected. In addition, etching residue and opening defect of a fine conduction hole which is smaller than a resolution power of an optical system cannot be detected. Further, a defect formed at a bottom portion of a step of a wiring pattern cannot be detected.
As described above, due to the miniaturization of circuit patterns, complexity of a circuit pattern shape and diversity of materials, defect detection by an optical image has become difficult. Therefore, a method of using an electron-beam image which is higher in resolution power than an optical image to perform a comparison inspection of a circuit pattern has been proposed,
When the comparison inspection of a circuit pattern is performed by using an electron-beam image, it is necessary to acquire the image much faster than the observation performed by scanning electron microscopy (hereinafter, abbreviated as SEM) so as to achieve a practical inspection time. Further, it is also necessary to secure the resolution power of the image acquired at a high speed and an SN ratio of the image. As a comparison-inspection apparatus for a pattern by using an electron beam, J.Vac. Sci. Tech.B, Vol. 9, No. 6, pp. 3005-3009 (1991) (Non-Patent Literature 2), J.Vac. Sci. Tech. B, Vol. 10, No. 6, pp. 2804-2808 (1992) (Non-Patent Literature 3), Japanese Patent Application Laid-Open Publication No. 5-258703 (Patent Literature 3), and a patent specification of U.S. Pat. No. 5,502,306 (Patent Literature 4) disclose a method in which an electron beam having electron-beam current equal to or more than one hundred times the ordinary SEM (10 nA or more) is irradiated to a conductive substrate (x-ray mask or the like), any of generated secondary electrons, reflection electrons and transmission electrons are detected, and comparison inspection of an image formed from signals corresponding to the detected electrons is preformed, thereby automatically detecting a defect.
As a method of inspecting or observing a circuit board having an insulator by an electron beam, Japanese Patent Application Laid-Open Publication No. 59-155941 (Patent Literature 5) and “ELECTRON, ION BEAM HANDBOOK” (THE NIKKAN KOGYO SHIMBUN. LTD.) pp. 622-623 (Non-Patent Literature 4) disclose a method in which a stable image is acquired by low-accelerative electron-beam irradiation equal to or less than 2 keV so as to reduce influence of charge.
Further, Japanese Patent Application Laid-Open Publication No. 2-15546 (Patent Literature 6) discloses a method of irradiating ion from the rear side of a semiconductor substrate, and Japanese Patent Application Laid-Open Publication No. 6-338280 (Patent Literature 7) discloses a method of irradiating light to a surface of a semiconductor substrate to cancel the charge to an insulator, respectively. Furthermore, it becomes difficult to acquire a high-resolution image due to space charge effect in a case of a large-current and low-accelerative electron beam. Therefore, as a method of solving this problem, Japanese Patent Application Laid-Open Publication No. 5-258703 (Patent Literature 3) discloses a method of decelerating a high-accelerative electron beam just before a sample and irradiating a substantially low-accelerative electron beam to the sample.
As a method of acquiring an electron-beam image at high speed, Japanese Patent Application Laid-open Publication No. 59-160948 (Patent Literature 8) and Japanese Patent Application Laid-Open Publication No. 5-258703 (Patent Literature 3) disclose a method of successively irradiating an electron beam to a semiconductor wafer on a sample table while continuously moving the sample table, thereby acquiring the electron-beam image. Also, as a detection apparatus of a secondary electron used in a conventional SEM, a structure including a scintillator (Al-evaporated phosphor), a light guide and a photoelectron multiplier tube is adopted. Since the detection apparatus of this type detects emission from a phosphor, frequency responsiveness is poor, and it is unsuitable for forming an electron-beam image at high speed. In order to solve this problem, as a detection apparatus for detecting a high-frequency secondary electron signal, detecting means using a semiconductor detector is disclosed in Japanese Patent Application Laid-open Publication No. 5-258703 (Patent Literature 3).